CtsNetUtils#storePrivateDnsSetting might require WRITE_SECURE_SETTINGS
permission to run.
Therefore, move it from setUp to only belong to the required test case.
Bug: 158538281
Test: run cts --include-filter "arm64-v8a CtsNetTestCases[instant]
android.net.cts.MultinetworkApiTest"
Change-Id: I8ecadd134824a4023a3c6e173d52ba088d46b103